Circuit interrupter



Feb. 10, 1953 c. L. STROUP 2,528,269

CIRCUIT INTERRUPTER Filed Sept. 26, 1947 2 SHEETS-SHEET l Cficz-rles Z fiiroup.

Feb. 10, 1953 c. STROUP 2,528,259

CIRCUIT INTERRUPTER Filed Sept. 26, 1947 2 SHEETS-SHEET 2 Cl/EEENT T/ME' MINUTES 5kV/7 TEn/VSFOEMEE 7200 y'qLTs To //0 220 VOL T5 Patented Feb. 10, 1953 UNITED STATES earsnr OFFICE CIRCUIT INTERRUPTER Charles L. Stroup, Oak Park, Ill.

Application September 26, 1947, Serial No. 776,34]

My invention relates, generally, to circuit inter rupters and it has particular relation to circuit interrupters for use with distribution type transformers for disconnecting the same from their high voltage energizing circuits.

Distribution transformers are employed in an electricity distribution system for stepping down transmission voltages of the order of 2300 volts and upwards to a relatively low voltage of the order of 110 to 220 volts. This low voltage is employed in the circuits which serve residential areas and the appliances normally used therein and like applications. These distribution transformers usually are located at the ends of feeder circuits which are commonly connected by suitable circuit interrupting means, such as a reclosing fuse or reclosing circuit breaker, to a common branch circuit which in turn is joined with other branch circuits forming a part of the distribution network of the utility system.

It is common practice to connect the primary Winding of each of the distribution transformers to the circuit feeding the same through a fuse. Various types of fuses are now available on the market and their use is well known. However, their range of operation is limited so that substantially the only function that they perform is to protect the transformer against a direct short circuit across the transformer primary or secondary terminals or a fault in one of the windings which results in the flow of a large amount of current.

Consider for the purposes of the present application that the distribution transformer has a capacity of 5 kva. and the feeder circuit thereto :2

has a voltage from line to ground single phase of 7200 volts. Th full load current of such a transformer is about 0.7 ampere. It is standard practice to employ a fuse having a 5 ampere rating for interconnecting such a transformer with its energizing circuit.

It often happens that a distribution transformer of this type is overloaded for short intervals of time. Such overloading is permissible if the condition does not exist long enough to impair the insulation of the transformer. Thus, it is standard practice to rate a transformer of this type so that it can withstand an overload of 100 per cent for a period of two hours and. a higher overload for a shorter period. Assuming that the transformer, above referred to, is operated at a 100 per cent overload for two hours, it is entirely possible that continued overload at this value for more than two hours would permanently injure the transformer Accordingly, it is desirable that r 5 Claims. (Cl. 175-294) it be disconnected from the energizing circuit in the event that the overload persists. A 100 per cent overload on a 5 kva. transformer, operating as above described, causes a current of only 1.4 amperes to flow in the primary winding. This current is substantially below the normal current rating of the 5 ampere fuse. As a result, it affords no protection in the event that the transformer is overloaded as described. As a matter of fact, even if the overload were doubled so that the transformer operated at a 300 per cent overload, the current flow in the primary winding would only be 2.8 amperes so that, even at this extreme overload, the 5 ampere fuse would not serve to interrupt the circuit.

Of course, it is possible to use a fuse having a smaller rating for protecting the transformer. For example, a fuse having a rating of l ampere could be used. In the event that the transformer were operated under 100 per cent overload conditions, such a fuse would not blow since a flow of 2 amperes is required to blow a 1 ampere fuse. However, it would blow under the 300 per cent overload condition with little time delay. Usually the rating of the transformer is such that it is capable of carrying a 300 per cent overload for relatively short periods without damage. The current-time characteristic of a fuse link is such that it would blow within a few seconds after the 300 per cent overload was applied to the transformer although the transformer could be safely operated for several minutes before it would be necessary to disconnect the same. In other words, the fuse does not follow the current-time characteristic of the transformer because, inherently, it has a relatively short current-time characteristic as compared with that of transformer.

Another disadvantage of employing a 1 ampere fuse in this situation is that it is more likely to be blown as a result of its being subjected to a lightning surge than in the case of the fuse of the 5 ampere rating. While the lightning surge may do no harm to the transformer, it might blow a l ampere fuse and thereby disconnect the transformer from the feeder circuit when such disconnection would serve no useful purpose.

It is often the case that a, bird or a squirrel or the like may cause a momentary short circuit across the terminals of the primary winding. Such a short circuit usually causes the blowing of even a 5 ampere fuse and most certainly of the l ampere fuse. Usually the short circuit is of a momentary nature and is promptly removed. However, the fuse may be blown and the service interrupted unnecessarily. It would be preferable course, it is not possible to do this when the conventional fuse of either the l ampere or the ampere rating is used.

In the event that the secondary winding of the transformer or the circuit connected thereto becomes short circuited, the impedance of the transformer is such that the current flowing in the primary winding under these conditions ordinarily does not exceed about 17 amperes. Such a current is sufilcient to blow either a l ampere fuse or a 5 ampere fuse. As a result, if either is used, the desired protection is not provided even though the secondary fault may be of a transient character. Thus, the transformer is quite likely to be disconnected under conditions where the same necessary.

Accordingly, among the objects of my invention are: To provide for interrupting the circuit to a load device, such as a transformer, in accordance with its current-time characteristic throughout its entire range; to control the functioning of a thermally rupturable element used for interrupting the current in this manner; to avoid the interruption of the circuit on the occurrence of a momentary short circuit, even across the load device terminals; to permit the load device to carry a substantial overload for a time within permissible limits and then to interrupt the circuit if the overload persists, the time being measured in a matter of minutes rather than seconds; to obtain a relatively large current for operating a relatively high current capacity thermally rupturable or fusible element connected in a circuit in which the normal full load current flow is relatively small so as to provide a relatively long time lag in its operation as compared to that of a relatively small capacity fusible element; and to avoid the unnecessary blowing of a thermally rupturable or fusible element by lightning surges or the like.

Other objects of my invention will, in part, be obvious and in part appear hereinafter.

For a more complete understanding of the nature and scope of my invention reference can be had to the following detailed description, taken together with the accompanying drawings, in which:

Figure 1 is a view, in side elevation, of a circuit interrupter constructed in accordance with my invention and located in operative position in a suitable mounting;

Figure 2 is a longitudinal sectional view, at an enlarged scale, of the circuit interrupter shown in Figure 1;

Figure 3 is a detail sectional view, at an enlarged scale, taken along the line 3--3 of Figure 2;

Figure 4 is a detail sectional view taken along the line i-fl of Figure 2;

Figure 5 illustrates diagrammatically the circuit connections that may be employed in practicing my invention; and

Figure 6 shows curves which demonstrate certain operating characteristics of my invention as compared to conventional fuse devices.

is entirely un- ,1.

Referring now particularly to Figure l of the drawings, it will be observed that reference character It designates an insulator which may be carried by a suitable mounting bracket II from a support such as the cross arm of a pole or the like. Terminals l2 and I3 are provided at the ends of the insulator in to permit connection of line conductors thereto. They may be held in place by suitable clamp nuts i l and 15 in conventional manner. Extending outwardly from the upper and lower ends of the insulator it are contact arms l8 and [1. As shown, the upper contact arm i6 is relatively rigid while the lower contact arm ll may be resilient or may be backed up by suitable spring members so as to bias the same away from the upper contact arm 16. It will be understood that the contact arms 26 and I7 may take various forms without departing from the scope of my invention.

At the outer ends of the contact arms IE and Il hook portions I3 and 19 are provided for receiving eyes 29 and 2! of a circuit interrupter, shown generally at 22, which is constructed in accordance with my invention.

As illustrated more clearly in Figure 5 of the drawings, the circuit interrupter 22 includes a thermally rupturable element 25 which, for present descriptive purposes, may be considered to be essentially the same as a ampere fuse in so far as its operating characteristics are concerned. The element 25 may be connected in series circuit relation with the primary winding 26 of a distribution transformer 2i that may be energized from a high voltage feeder circuit represented by the conductors 28. The transformer 2? has a secondary winding 29 which may be connected to a low voltage load circuit represented by the conductors 3d. The load circuit may be or the conventional -220 volt type for supplying residential loads and the like.

For illustrative purposes, the transformer 2? is indicated as being a 5 kva. transformer arranged to be connected to the circuit represented by the conductors 23 energized at a voltage of 7200 volts. The normal full load current of such a transformer is 0.7 ampere. Of course, it will be understood that my invention, appropriately modified, can be employed in conjunction with transformers having other ratings, these particular values being selected for illustrative purposes only.

As indicated, the current rating of the thermally rupturable element 25 is substantially in excess of the normal current carrying capacity of the circuit to and including the primary winding 28. It follows that it would not be operated solely as the result of the current flowing therethrough to the primary winding 26 under any circumstances. Because of this it is possible to obtain the desired current-time characteristic in the operation of the thermally rupturable element 25 by the means which will be described presently.

With a View to rupturing the element 25 under certain desired conditions it is connected across a secondary winding 32 which comprises a single turn that is formed by a generally O-shaped copper conductor 33, Figures 2 and. 4;, together with the element 25. The single turn secondary winding 32 form a part of a current transformer that is indicated, generally, at 35 and has a core 35 of magnetic material that is formed preferably by spirally wrapping a suitable length of transformer steel to form a toroidal core. A

primary winding 36 surrounds the toroidal core 35. For illustrative purposes it is pointed out that the primary winding 36 may have 100 turns of number copper wire. The size and character of the toroidal core 35 are such that it becomes saturated on flow of about 50 amperes through the primary winding 36. The maximum current that can flow through the single turn secondary winding 32 is about 300 amperes, this amount being determined by the saturation of the toroidal core 35. Thus, regardless of how much current in excess of 50 amperes flows through the primary winding 36, there is no corresponding increase in the flow of current through. the single turn secondary winding 32.

It will be noted that the primary winding 36 of the current transformer 34 is connected in series circuit relation with the thermally rupturable element so that the current flowing through the former also flows through the primary winding 26 of the distribution transformer 21. This current also flows through the thermally rupturable element 25 but, as described hereinbefore, it has substantially no efiect on its operating characteristics since, in the range of operating conditions to which the element 25 normally will be subjected. the current flowing through the primary windings 2t and 35 can be neglected as compared to the current which flows through the element 25 from the single turn secondary winding 32.

Referring now particularly to Figures 2 and 4 of the drawings, it will be observed that the conductor 33 is slotted as indicated at 3! in the upper and lower sides and that it has apertures 38 therein for frictionally engaging enlarged ends 39 and 40 of a link 41 which forms a part of the thermally rupturable element 25. Screws 42, extending transversely through the upper and lower sides of the conductor 33, serve to grip the ends 39 and 4% of the link 4! to provide the desired contact engagement therebetween having a relativel low resistance. If desired, the ends 39 and Ml of the link 4! can be threaded with corresponding threads being provided internally of the apertures 38. in either case it is desired to provide a minimum of contact resistance between the ends 39 and 48 of the link ti and the generally O-shaped conductor 33 in order to keep the resistance of the secondary winding of the current transformer 3 1 to a minimum.

As described hereinbefore, the primary winding 36 of the current transformer 34 is connected in series circuit relation between one of the high voltage feeder circuit conductors 28 and the.

primary winding 26 of the distribution transformer 2?. For this purpose conductors 43 and 44 may be provided. The conductor 43 serve to connect the primary winding 35 to the upper 28 of the circuit interrupter 22 which, it will be recalled, is carried by and connected to the upper contact arm it. The ccnductcr 43 may have return bend portion to facilitate removal or the current transformer 34 from the housing in which it is contained to permit removal and replace ment of the element 25. The other conductor 34 may be connected directly to the Q-shaped conductor 33 where it is connected to the lower eye 2! by means new to be described.

It will be observed that the link 41 comprises an elongated member that is preferably formed of conducting material such as brass and that it functions as a heater element. It ha a longitudinal aperture d5 that extends from the lower end upwardly to about the central section and within this aperture 45 there is located a conductor 46. The conductor 46 may comprise a rigid member such as a length of number 20 copper wire. If desired, it may be a flexible conduct-or formed of a larger number of strands of relatively fine wire having a conductivity equivalent to that of a number 29 copper conductor.

The conductor 16 preferably is held in place within the link 4% by a low melting point metal or alloy 4? such as solder, Woods metal, and the like. This is shown more clearly in Figure 3. The low melting point metal 4'! may be inserted in the molten state through a transverse aperture 43. Thus, it will be observed that the low melting point metal 41 holds the conductor 46 in place at the junction of the apertures 45 and 48. It is the melting of this low melting point metal 4! which determines the operating characteristics of the element 25. The heat for melting the metal 47 is generated by the current flowing through the link or heater 4i. Because of the relatively high thermal capacity of the link 4| and the o shaped conductor 33, which together make up the single turn secondary winding 32 of the current transformer 3Q, it is possible to provide the desired time lag in the melting of the metal 47. Moreover, because of the relatively massive physical character of these parts, they have relatively great current carrying capacity so that the low melting point metal d! is unlikely to be affected by current flowing therethrough which also flows through the primary windings 26 and $5 of the transformers 2! and 3 5.

When the metal 41' i melted as the result of sufiicient heat having been generated by the link 45, the conductor 68 is released and the resilient contact arm i? whips the same downwardly through an arcing tube at that may be formed of a fiber sleeve or formed of similar material which. evolves an arc extinguishing medium when it is subjected to the heat of an are. It will be observed that the sleeve lt fits over the reduced lower end 5!! of the link M. It may be held in place there by suitable means as desired. As illustrated in Figure 2, the current transformer 3 can be located within a generally cylindrical housing 5| that opens downwardly and is secured centrally to a shank 52 which extends upwardly for engagement with the lower portion of the eye 29. If desired, the shank 52 can be a flexible conductor rather than a rigid member as shown. The housing 5| may be formed of metal or insulation as desired. It serves to protect the current transformer 34 from the weather. At the lower end of the housing 5| there is provided a support ring 53 of suitable insulation which carries the current transformer 34 and which may be secured in place by laterally extending screws 54 projecting through the housing 5|. An insulating ring 55 surrounds the primary winding 36 of the current transformer 34 and serves to insulate the same from the housing 5| and to maintain the desired spaced relation. If desired, the insulating ring 55 can be omitted where the housing is formed of insulating material.

In Figure 6 of the drawings, the curve 55 represents the current-time characteristic of a transformer, such as the 5 kva. transformer described hereinbefore. In this curve the time is indicated in units of minutes rather than seconds as is the usual practice where the operating characteristics of conventional fusible elements are plotted. The current-time characteristic 5% illustrates the amount of current that can be carried for a given time interval without the rating of the transformer being exceeded.

As indicated hereinbefore, it is desirable to provide for protecting the transformer 27 with a circuit interrupter which has a current-time characteristic that is essentially the same as that illustrated by the curve 56. The vurve 51 illustrates the current-time characteristic of a conventional fusible element which relies for its operation on the current flow therethrough which is identical with the current flowing through the distribution transformer primary winding 26. Because of the inherent characteristics of such a device, it is not possible to construct it so that it even approaches the shape of the current-time characteristic 56 of the transformer.

However, when the circuit interrupter 22, constructed as set forth hereinbefore, is employed it is possible to obtain with it a current-time characteristic which may be represented by the curve 58. It will be observed that the curve 58 is less by substantially the same amount than the characteristic curve 56 for the transformer '21 throughout its entire extent. The reason for constructing the circuit interrupter 22 so that it will have a current-time characteristic uniformly spaced from the curve 56 is to insure that the transformer 27 is not permitted to operate along the curve 56 which is considered to represent the extreme limit of its operating characteristics. In other words, in order to give a certain margin of safety the circuit interrupter 22 is so designed that its characteristic curve 58 always is spaced from the characteristic curve 56 of the transformer 2'! in the direction indicated in Figure 6.

In the event that the secondary winding 26 of the transformer 21 should become short circuited momentarily, as by the conductors 36 swinging together into momentary contact, the current flowing through the primary winding 26 under these assumed conditions and with the transformer 21 being as described hereinbefore is about 17 amperes. This current is suflicient to promptly blow the conventional fuse which might be connected between the primary winding 26 and the high voltage feeder circuit 28. This is illustrated by the characteristic curve 51 of such a device as shown in Figure 6. Since such a short circuit in the secondary winding of the transformer 21 is of a transient character, it is not desired that the circuit to the primary winding 26 be opened.

Accordingly, when the transformer is protected by the circuit interrupter 22, it will be obvious that this amount of current flowing through the element 25 will not be sufficient to cause the same to operate. Because of its inherent thermal lag, the metal 4'! will not be heated sumciently to release the conductor 46 and open the circuit when the short circuit on the secondary winding of the transformer 21 is of the character described. On the other hand, should the short circuit on the secondary winding of the transformer 21 persist, the metal 41 will be heated suficiently to release the conductor 46 and open the primary circuit within a relatively short space of time, which, as shown by the curve 58, is less than the time required to damage the transformer 21 under these fault operating conditions.

As indicated hereinbefore, it is often the case that a bird or squirel may short circuit momentarily the terminals of the primary winding 26. In such case, if the transformer 21 is protected by a conventional 'fuse, the fuse will be blown promptly although, if it had not blown, the bird or squirrel might have dropped off the transformer, thereby rendering it unnecessary to open the circuit. Or, if the circuit were not opened by the fuse, sufiicient current might flow to burn up the bird or animal causing the short circuit. Alternatively, sufficient current might flow to cause the reclosing fuse or breaker which is common to several feeder circuits, to open the circuit momentarily and thus permit the bird or animal to drop away from the circuit.

Where the circuit interrupter 22 is employed, it will be obvious that the circuit will not be interrupted under these conditions by the operation of the element 25 since it requires an appreciable time to bring it up to the temperature that will effect the melting of the metal 41. At the same time, should the fault condition persist sulficiently long enough, the element 25 eventually will operate to clear the circuit. In the meantime, however, an opportunity has been given for the dropping away of the bird or animal causing the short circuit for one reason or another.

After the circuit interrupter 22 has operated, it may be removed from the upper contact arm 16 by a switch stick. The ring 53 is removed and the current transformer is dropped out of the housing 5|. The screws 42 are loosened and the I blown link 4| is removed and replaced by a new link with the conductor 46 held in place by the metal 4'1. The interrupter then can be replaced in the circuit by the switch stick.

It will be seen now that the circuit interrupter 22, constructed as described herein, serves to provide the transformer 21 with a degree of protection which has not heretofore been possible. At the same time, it permits it to operate under various degrees of overload without unnecessarily disconnecting the same from the circuit. Moreover, it permits of a greater continuity in the energization of the transformer 21 in the event that the terminals of the high voltage primary Win-ding 26 are momentarily short circuited. Likewise, it gives protection in the event that the secondary winding 29 is short circuited for an appreciable length of time but does not disconnect the transformer 2'! in the event that the fault on the secondary circuit is of a momentary nature.

Since further changes can be made in the foregoing construction and different embodiments of the invention can be made without departing from the spirit and scope thereof, it is intended that all matters shown in the accompanying drawings and set forth hereinbefore shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In a protective device of the class described, in combination, a thermally rupturable fuse element for connection in series with the primary winding of a distribution transformer and adapted when thermally ruptured on the occurrence of a fault in the transformer or in the circuit connected to its secondary winding directly to disconnect the primary winding of the distribution transformer from a high voltage energizing circuit, said fuse element having a current carrying capacity substantially greater than the dangerous load current rating of the primary winding of the distribution transformer, a current transformer having a primary winding connected in circuit with said fuse element for connection of both said fuse element and the primary winding of said current transformer in series with the nected to its secondary winding for a time ini terval in excess of that which would be required thermally to rupture a fuse connected in series with the primary winding of the distribution transformer and having a current carrying capacity less than the dangerous load current carrying capacity of the distribution transformer, the current transformer having a generally annular core with the secondary Winding therefor comprising a first conductor having at least one portion extending longitudinally along the outer side J- of the core and inturned end portions extending in generally parallel relation along the opposite ends of the core, the metallic heater link extending generally axially within the core and having contact with the inturned portions of the first conductor which constitutes the secondary winding at axially spaced locations, and a second conductor held to the heater link by a fusible element of a character to be thermally ruptured by heat generated in said heater link by the current which is induced by the primary winding of the current transformer in the first conductor which constitutes the secondary Winding and which flows through the heater link.

2. In a protective device of the class described,

in combination, a thermally rupturable fuse element for connection in series with the primary winding of a distribution transformer and adapted when thermally ruptured on the occurrence of a fault in the transformer or in the circuit connected to its secondary winding directly to disconnect the primary winding of the distribution transformer from a high voltage energizing circuit, said fuse element having a current carrying capacity substantially greater than the 1 dangerous current rating of the primary winding of the distribution transformer, a current transformer having a primary winding connected in circuit with said fuse element for connection of both said fuse element and the primary winding of said current transformer in series with the primary winding of the distribution transformer,

' said current transformer having a secondary winding, a metallic heater link connected in circuit with the secondary winding of the current transformer and positioned adjacent to aid fuse element, said current transformer and heater link being of a character whereby thermally to rupture said fuse element on duration of a fault in the distribution transformer or in the circuit connected to its secondary winding for a time interval in excess of that which would be required thermally to rupture a fuse connected in series with the primary winding of the distribution transformer and having a current carrying capacity less than the dangerous load current carrying capacity of the distribution transformer, the current transformer having a generally annular core with the secondary winding therefor comprising a first conductor having at least one portion extending longitudinally along the outer side of the core and inturned end portions extending in generally parallel relation along the opposite ends of the core, the metallic heater link extending generally axially within the core and 10 having contact with the inturned portions of the first conductor which constitutes the secondary winding at axially spaced locations, a second conductor held to the heater link by a fusible element of a character to be thermally ruptured by heat generated in said heater link by the current which is induced by the primary winding of the current transformer in the first conductor which constitutes the secondary winding and which flows through the heater link, the heater link having a tubular end within which the adjacent end of the second conductor is held by the fusible element, and an arcing tube secured telescopically over the tubular end of the heater link and projecting over the adjacent end of the second conductor.

3. In a protective device of the class described, in combination, a thermally rupturable fuse element for connection in series with the primary winding of a distribution transformer and adapted when thermally ruptured on the occurrence of a fault in the transformer or in the circuit connected to its secondary winding directly to disconnect the primary winding of the distribution transformer from a high voltage energizing circuit, said fuse element having a current carrying capacity substantially greater than the dangerous load current rating of the primary winding of the distribution transformer, a current transformerhaving a primary winding connected in circuit with said fuse element for connection of both said fuse element and the primary winding of said current transformer in series with the primary winding of the distribution transformer, said current transformer having a secondary winding, a metallic heater link connected in circuit with the secondary winding of the current transformer and positioned adjacent to said fuse element, said current transformer and heater link being of a character whereby thermally to rupture said fuse element on duration of a fault in the distribution transformer or in the circuit connected to its secondary winding for a time interval in excess of that which would be required thermally to rupture a fuse connected in series with the primary winding of the distribution transformer and having a current carrying capacity less than the dangerous load current carrying capacity of the distribution transformer, the current transformer having a generally annular core with the secondary winding therefor comprising a first conductor having at least one portion extending longitudinally along the outer side of the core and inturned end portions extending in generally parallel relation along the opposite ends of the core, the metallic heater link extending generally axially within the core and having contact with the inturned portions of the first conductor which constitutes the secondary winding at axially spaced locations, a second conductor held to the heater link by a fusible element of a character to be thermally ruptured by heat generated in said heater link by the current which is induced by the primary winding of the current transformer in the first conductor which constitutes the secondary winding and which flows through the heater link, the heater link having a tubular end within which the adjacent end of the second conductor is held by the fusible element, an arcing tube secured telescopically over the tubular end of the heater link and projecting over the adjacent end of the second conductor, a housing enclosing the primary and secondary windings of the current transformer and the heater link, said housing having an end wall, and a terminal link carried by and projecting outwardly from said end wall in substantial alignment with the heater link and the second conductor, the second conductor constituting a second terminal link and the primary winding of the current transformer being connected in series with said terminal links and said heater element.

4. In a protective device of the class described, in combination, a thermally rupturable fuse element for connection in series with the primary winding of a distribution transformer and adapted when thermally ruptured on the occurrence of a fault in the transformer or in the circuit connected to its secondary winding directly to disconnect the primary winding of the distribution transformer from a high voltage energizing circuit, said fuse element having a current carrying capacity substantially greater than the dangerous load current rating of the primary winding of the distribution transformer, a current transformer having a primary winding connected in series with said fuse element for connection of both said fuse element and the primary winding of said current transformer in series with the primary winding of the distribution transformer, said current transformer having a secondary winding, a metallic heater link connected in circuit with the secondary winding of the current transformer and positioned adjacent to said fuse element, said current transformer and heater link being of a character whereby thermally to rupture said fuse element on duration of a fault in the distribution transformer or in the circuit connected to its secondary winding for a time interval in excess of that which would be required thermally to rupture a fuse connected in series with the primary winding of the distribution transformer and having a current carrying capacity less than the dangerous load current carrying capacity of the distribution transformer, the current-time characteristic of the fuse element being less by substantially the same amount than that of the distribution transformer, the metallic heater link being connected across the secondary winding of the current transformer, and a conductor for connection into the circuit of the primary winding of the distribution transformer and held to the metallic heater link by the fuse element, said conductor being releasable from the heater link on rupture of the fuse element to draw an arc therebetween while the heater link maintains a short circuit across the secondary winding of the current transformer.

5. In a protective device of the class described, in combination, a thermally rupturable fuse element for connection in series with the primary winding of a distribution transformer and adapted when thermally ruptured on the occurrence of a fault in the transformer or in the circuit connected to its secondary winding directly to disconnect the primary winding of the distribution transformer from a high voltage energizing circuit, said fuse element having a current carrying capacity substantially greater than the dangerous load current rating of the primary winding of the distribution transformer, a current transformer having a primary winding connected in series with said fuse element for connection of both said fuse element and the primary winding of said current transformer in series with the primary winding of the distribution transformer, said current transformer having a secondary winding, a metallic heater link connected in circuit with the secondary winding of the current transformer and positioned adjacent to said fuse element, said current transformer and heater link being of a character whereby thermally to rupture said fuse element on duration of a fault in the distribution transformer or in the circuit connected to its secondary winding for a time interval in excess of that which would be required thermally to rupture a fuse connected in series with the primary winding of the distribution transformer and having a current carrying capacity less than the dangerous load current carrying capacity of the distribution transformer, the current-time characteristic of the fuse element being less by substantially the same amount than that of the distribution transformer, the metallic heater link being connected across the secondary winding of the current transformer and having a tubular end, and a conductor for connection into the circuit of the primary winding of the distribution transformer and extending into the tubular end of the heater link and held to the heater link within the tubular end thereof by the fuse element, said conductor being releasable from the heater link on rupture of the fuse element to draw an arc therebetween while the heater link maintains a short circuit across the secondary Winding of the current transformer.

CHARLES L. STROUP.

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